$10 to be honest seems very very small, considering what they put into the expansion. $20 does not seem unreasonable.
However i wish the game was a single game, by that what i mean is the expansion overwrites the main folder and there is no Vanilla AND Expansion, which effectively splits the community. But from a dev standpoint having those who buy the expansion still able to play with those who haven't is a move to keep the community together in the on-line games. That is my take on why they did it that way, just a guess on my part, so maybe im wrong. But it seems a reasonable reason.
I am not a developer by any means and so my understanding of what works for a gaming communtiy is rather obviously amateurish, but if Vanilla were to move by closer degrees toward what Entrenchment is, file system, file format, single game folders (which is where it seems the devs are in fact taking it) then the Expansions could add small differences, new skyboxes, additional ships, starbases, etc... which might of course mean that players without the updates can in fact play against, (and probably lose to) players that have the starbases and the added ships and perks of the game. It would mean the players without could "see" the starbases and other ships, the cool new skyboxes and neat abilities, but not have access to them of course.
It might mean people saying "Hey, i want a starbase too!!" Or i woulda whooped him if i had all those structure killer ships too!"
Just two cents worth of thoughts, and perhaps not even workable, but just an idea.

-Teal